Refactor git version detection for reuse

This way we can use it to detect feature support in the underlying
git, such as new options or commands that have been added in more
recent versions.

Signed-off-by: Shawn O. Pearce <sop@google.com>

git_command.py

@@ -68,6 +68,30 @@ class _GitCall(object):
     return fun
 git = _GitCall()
 
+_git_version = None
+
+def git_require(min_version, fail=False):
+  global _git_version
+
+  if _git_version is None:
+    ver_str = git.version()
+    if ver_str.startswith('git version '):
+      _git_version = tuple(
+        map(lambda x: int(x),
+          ver_str[len('git version '):].strip().split('.')[0:3]
+        ))
+    else:
+      print >>sys.stderr, 'fatal: "%s" unsupported' % ver_str
+      sys.exit(1)
+
+  if min_version <= _git_version:
+    return True
+  if fail:
+    need = '.'.join(map(lambda x: str(x), min_version))
+    print >>sys.stderr, 'fatal: git %s or later required' % need
+    sys.exit(1)
+  return False
+
 class GitCommand(object):
   def __init__(self,
                project,